Surface light source device, and liquid crystal display device, sign display apparatus and traffic sign display apparatus using the surface light source device

ABSTRACT

A surface light source device, comprising a light source ( 2 ); a light conductor ( 1 ) which has a light incident face ( 11 ) on at least one side end surface thereof which confronts the light source ( 2 ), and a light emitting face ( 12 ) on one surface thereof which is substantially perpendicular to the light incident face ( 11 ); and a light angle varying sheet ( 3 ) which is disposed at a side of the light emitting face ( 12 ) of the light conductor ( 1 ), wherein at least one of the light emitting face ( 12 ) and a back surface ( 13 ) of the light conductor ( 1 ) comprises a minute structure having an average slant angle of 0.5 to 7.5 degrees. The light angle varying sheet ( 3 ) may comprise a prism sheet having a plurality of prisms ( 31 ) which are formed parallel to one another on at least one surface thereof. The minute structure may comprise a roughened surface which includes a plurality of fine convex members each having a substantially spherical surface or a plurality of prism arrays having slant surfaces which extend parallel to said light incident face ( 11 ) and which have an average slant angle of 0.5 to 7.5 degrees.

This Application is a Divisional of Ser. No. 09/461,342 filed Dec. 15,1999, U.S. Pat. No. 6,244,719 which is a continuation of Ser. No.09/117,505 filed Jul. 30, 1998 U.S. Pat. No. 6,099,135, which is a 371of PCT/JP97/00237 filed Jan. 31, 1997.

FIELD OF THE INVENTION

The present invention relates to a surface light source device for adisplay apparatus such as a liquid crystal display device for use in aportable personal computer, a liquid crystal television or the like; asign display apparatus such as a guide marking board, a large-sizesignboard or the like used in a station, public facilities or the like;or for a traffic sign display apparatus such as various types of guidesigns, traffic signs or the like on a highway road or a general road;and the present invention relates more particularly to a surface lightsource element for emitting light which has high brightness and auniform brightness distribution on a light emitting plane withoutperforming any uniformity-enhancing processing such as treatment with aspot pattern or the like.

DESCRIPTION OF THE RELATED ART

Recently, a color liquid crystal display device has been widely used invarious applications such as in portable personal computers, liquidcrystal televisions, video built-in type liquid crystal televisions,etc. This liquid crystal display device comprises a back light portionand a liquid crystal display portion. An under-lighting system in whicha light source is disposed just under the liquid crystal display portionand an edge-lighting system in which a light source is disposed on theside surface of a light conductor are used as a lighting system for theback light portion.

Recently, the edge-lighting system has been more frequently used becauseit is more suitable for reducing the size of the liquid crystal displaydevice. In the edge-lighting system, the light source is disposed at aside surface portion of a planar light conductor so as to emit lightfrom the entire surface of the light conductor, and thus the back lightportion using this system is called a “surface light source device”.

According to such a surface light source device, a light conductor isformed of a planar transparent member such as an acrylic resin plate orthe like, and light emitted from a light source which is disposed at theside surface of the light conductor is introduced through the sidesurface (light incident face) of the light conductor into the lightconductor. The incident light totally (completely) reflects from theobverse surface of the light conductor (light emitting face) and theback surface of the light conductor and then passes through the lightconductor. Further, a light emitting function or light emitting portionsuch as a light scattering portion is provided on the obverse surface orback surface of the light conductor to emit the light from the wholelight emitting face. However, when the light emitting portion isuniformly formed on the obverse surface or the back surface of the lightconductor, the brightness of the emitted light is more reduced as thelight is farther away from the light source, so that the brightnessdistribution on the light emitting plane becomes disuniform and thus ahigh-quality display image cannot be obtained.

This result is more noticeable as the size of the liquid crystal displaydevice increases, and thus the surface light source device cannotpractically be used for a large-size liquid crystal display device of10-inches or more in size. Despite this, large-size liquid crystaldisplay devices have recently been in demand and, following this recentdemand, liquid crystal display devices used for portable personalcomputers, liquid crystal televisions or the like have additionally beenrequired to have a brightness distribution of very high uniformity onthe screen thereof.

Furthermore, marking apparatuses such as a guide signboard or alarge-size signboard, and traffic sign apparatuses such as a guide sign,a traffic signboard or the like, have used two illumination systems,i.e., an internal illumination system and an external illuminationsystem, to enhance visual recognition and character recognition atnight. According to the internal illumination system, characters,figures, photographs, etc., are formed on a semi-transparent plasticplate such as a methacrylate plate or the like by cut-out, print or thelike to form a display plate. A light source is disposed at the insideof the display plate, and the display plate is illuminated by the lightsource. A rod (linear pipe shape) or annular type fluorescent lamp isgenerally used as the light source. According to the externalillumination system, a light source is disposed at any of the upper andlower sides or right and left sides of a display plate on which aninformation display is formed, and the whole surface of the displayplate is illuminated by the light source. A rod type fluorescent lamp isgenerally used as the light source.

In the conventional display devices as described above, the brightnessdistribution on the entire surface of the display plate is disuniform,that is, the ratio of the maximum value/minimum value of the brightnessis very large. Therefore, it is very difficult to provide a displaydevice having an uniform brightness distribution by using theseillumination systems. In particular, this problem is more serious forthe external illumination system. Further, the internal illuminationsystem has another problem in that a fluorescent lamp or the like whichis used as the light source can be unintentionally seen through thedisplay plate (i.e., a see-through phenomenon occurs).

Therefore, attempts have been made to apply the edge lighting type backlight system in which a light source is disposed at the side surfaceportion of a planar light conductor to emit light from the entiresurface of the light conductor to the display devices as describedabove. However, these display devices need a large-size surface lightsource device, and thus they have the same problem as the liquid crystaldisplay device in that sufficient uniformity in brightness cannot beobtained within the light emitting face.

In order to solve this problem of “disuniformity of brightness” of thesurface light source device, various proposals have been made. Forexample, Japanese Laid-open Patent Application No. Hei-1-245220 proposesa surface light source device having a light emitting portion which isobtained by coating or sticking light scattering material to the backsurface confronting the light emitting face of the light conductor sothat the density of the light scattering material increases withincreasing distance from the light incident face. Further, JapaneseLaid-open Patent Application No. Hei-1-107406 proposes a light conductorcomprising plural laminated transparent plates on which fine spotsformed of light scattering material are formed in various patterns. Insuch a surface light source device, since white pigment such as titaniumoxide, barium sulfate or the like is used as the light scatteringmaterial, optical loss occurs due to light absorption or the like whenthe light impinging against the light scattering material is scattered.Therefore, although uniformity of the brightness distribution can beachieved, the brightness of the emitted light is reduced.

Further, Japanese Laid-open Patent Application No. Hei-1-244490 andJapanese Laid-open Patent Application No. Hei-1-252933 propose a surfacelight source device in which an emitted light adjusting member or alight diffusion plate having a light reflection pattern which is matchedto the reciprocal of a light emission distribution is disposed on thelight emitting face of the light conductor. However, in such a surfacelight source device, since the light reflected from the emitted lightadjusting member or the light diffusion plate can not be reused, thesame optical loss also occurs. Therefore, the brightness of the emittedlight in a desirable direction is reduced.

Still further, Japanese Laid-open Patent Application No. Hei-2-17 andJapanese Laid-open Patent Application No. Hei-2-84618 propose a surfacelight source device in which a satin-finished face is or many lens unitsare formed on at least one of the light emitting face or the backsurface confronting the light emitting face of the light conductor, anda prism sheet is mounted on the light emitting face. In such a surfacelight source device, the brightness of the emitted light is very high,however, the uniformity of the brightness distribution on the lightemitting face is still unsatisfactory. Therefore, this type of surfacelight source device is practically usable as only a small-size surfacelight source element in a size of several inches.

In order to provide a surface light source device, which can achieveuniformity in brightness of emitted light and reduce the optical loss toenhance the brightness, Japanese Laid-open Patent Application No.Hei-6-18879 proposes the following surface light source device. In thissurface light source device, a satin-finished face is or many lens unitsare formed on the light emitting face of the light conductor, aroughened surface portion and a flat surface portion are formed on theback surface of the light conductor so that the ratio of the roughenedsurface portion to the flat surface portion increases with increasingdistance from the light source, and a prism sheet is mounted on thelight emitting face. In this surface light source device, the uniformityof the brightness distribution of the emitted light can be achieved andthe optical loss can be reduced. However, when the surface light sourcedevice is used for a display device such as a liquid crystal displaydevice, a marking apparatus or the like, a pattern which is formed ofthe roughened surface portion and the flat surface portion on the backsurface of the light conductor can be observed through the liquidcrystal display panel or the display plate, which prevents a viewer fromseeing an image.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a surfacelight source device for emitting light which has high brightness andhigh uniformity in brightness distribution within a light emitting facewithout performing a uniformity enhancing treatment with a spot patternor the like.

In view of the foregoing situation, the inventors of the presentapplication have made various earnest studies on the structure of thelight emitting face and the back surface of a light conductor, andthrough these studies they have found out that a surface light sourcedevice which can emit light having high brightness and high uniformityof brightness distribution within a light emitting face withoutperforming a uniformity enhancing treatment with a spot pattern or thelike can be provided by designing the light emitting face or the backsurface thereof to have a roughened surface having a fine uneven shapewith a specific average oblique angle, or to have an uneven surfacecomprising a plurality of lens arrays with a specific average obliqueangle.

That is, a surface light source device according to the presentinvention comprises:

a light source;

a light conductor which has a light incident face on at least one sideend surface thereof which confronts the light source, and a lightemitting face on one surface thereof which is substantiallyperpendicular to the light incident face; and

a light angle varying sheet which is disposed at a side of the lightemitting face of the light conductor,

wherein at least one of the light emitting face and a back surface ofthe light conductor comprises a minute structure having an average slantangle of 0.5 to 7.5 degrees.

A surface light source device according to a first aspect of the presentinvention comprises: a light source; a light conductor which has a lightincident face on at least one side end surface thereof confronting thelight source and a light emitting face on one surface thereof which issubstantially perpendicular to the light incident face; and a lens sheetwhich is disposed at the light emitting face side of the light conductorand has a plurality of parallel lens arrays on at least one surfacethereof, wherein at least one of the light emitting face and the backsurface of the light conductor comprises a roughened surface whichincludes a plurality of fine convex members each having a substantiallyspherical surface, and the average oblique angle of the roughenedsurface is set to 0.5 to 7.5 degrees.

Furthermore, a surface light source device, according to a second aspectof the present invention includes a light source, a light conductorwhich has a light incident face on at least one side end surface thereofconfronting the light source and a light emitting face on one surfacethereof which is substantially perpendicular to the light incident face,and a lens sheet which is disposed at the light emitting face side ofthe light conductor and has many parallel lens arrays on at least onesurface thereof, wherein at least one of the light emitting face and theback surface of the light conductor comprises many lens arrays whichextend in parallel to the light incident face and each have slantsurfaces having an average oblique angle of 0.5 to 7.5 degrees.

Still furthermore, each of a liquid crystal display device, a signdisplay apparatus and a traffic sign display apparatus according to thepresent invention uses the surface light source device as a back light.

According to the present invention, many fine convex members which havethe substantially spherical surface and the average oblique angle of 0.5to 7.5 degrees are formed on at least one of the light emitting face andthe back surface confronting the light emitting face of the lightconductor, or many lens arrays comprising slant surfaces having theaverage oblique angle of 0.5 to 7.5 degrees are formed in parallel tothe light incident face on at least one of the light emitting face andthe back surface of the light conductor. With this construction, thelight emission efficiency of the light emitted from the light emittingface of the light conductor can be reduced, thereby enabling greateramount of light to propagate toward the tip end portion of the lightconductor. Therefore, the high uniformity of the brightness within thelight emitting face can be achieved without performing the uniformitytreatment using the spot pattern or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a surface light source deviceaccording to the present invention;

FIG. 2 is a schematic diagram showing an optical path of light on alight emitting face of a light conductor according to the presentinvention;

FIG. 3 shows a coordinate system in which the spherical shape of aconvex member according to the present invention is simplified as acircle;

FIG. 4 is a partial cross-sectional view showing a prism surface of thelight conductor according to the present invention;

FIG. 5 is a partial cross-sectional view showing a lenticular lenssurface of the light conductor according to the present invention;

FIG. 6 is a side view showing a light conductor of the surface lightsource device according to present invention;

FIG. 7 is a side view showing another light conductor of the surfacelight source device according to present invention;

FIG. 8 is a partial perspective view showing a liquid crystal displaydevice according to present invention;

FIG. 9 is a graph showing a distribution of light emitted from the lightconductor;

FIG. 10 is a chart showing the surface roughness of a roughened surfaceof a device of Example 1 according to the present invention, and showingprimary and secondary differentials of the surface roughness;

FIG. 11 is a chart showing the surface roughness of a roughened surfaceof a device of Comparative Example 1 of the present invention, andshowing primary and secondary differentials of the surface roughness;

FIG. 12 is a partial cross-sectional view showing a prism surface of alight conductor of a comparative example; and

FIG. 13 is a partial cross-sectional view showing a lenticular lenssurface of the light conductor of another comparative example.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a perspective view showing a surface light source device of anembodiment according to the present invention. As shown in FIG. 1, thesurface light source device of this embodiment includes an elongatedlight source 2, a light conductor 1 which has at least one lightincident face 11 confronting the light source 2 and a light emittingface 12 which is substantially perpendicular to the light incident face11, and a light angle varying sheet 3 comprising a lens sheet such asprism sheet mounted on the light emitting face 12 of the light conductor1. In the surface light source device thus constructed, a part of thelight which is emitted by the light source 2 and incident onto the lightconductor 1, and which has an incident angle distribution over acritical angle, propagates in the light conductor 1 while being totallyreflected repetitively from the light emitting face 12 and a backsurface 13 of the light conductor 1. When the surface (the lightemitting face 12) of the light conductor 1 is designed to be uneven,light which reaches an uneven portion at an angle below the criticalangle with respect to the uneven portion is refracted from the unevenportion and emitted to the outside of the light conductor 1. On theother hand, light which reaches an uneven portion at an angle exceedingthe critical angle is totally reflected from the uneven portion andcontinues to propagate in the light conductor 1. This phenomenon happensbecause the light traveling direction (i.e., whether the light isreflected or refracted) is determined according to Snell's law. In otherwords, it is determined by the refractive index of a medium and thelight incident angle with respect to the normal of the surface to whichthe light is incident.

FIG. 2 is a schematic diagram showing the light refraction andreflection in the light conductor 1 having an uneven portion on thesurface thereof. Light A which is incident onto a slant surface of theuneven portion at an incident angle i, which is below the criticalangle, is emitted from the light conductor 1 at an refraction angle i′which satisfies the relationship of nsin(i)=sin(i′) (n represents therefractive index of the light conductor 1) according to Snell's law. Onthe other hand, light B which is incident onto the slant surface at anangle k, which exceeds the critical angle, is totally reflected from theslant surface at an angle k′ (k′=k), and continues propagating in thelight conductor 1. The light which has been once incident onto theuneven portion and then reflected therefrom is liable to change anincident angle when it is incident again onto the uneven portion, sothat there is a probability that the light will be emitted to theoutside of the light conductor 1.

The inventors of this application have experimentally found out that therelationship between the light emission intensity (I) at a point and thelight emission intensity (I₀) at the light incident face end of thesurface light source device satisfies the following equation (1):

I=I ₀(1−α/100)^(L′/t)   (1)

where α represents the light emission rate, L′ represents the distancefrom the light incident face end and t represents the thickness of thelight conductor 1.

It is apparent from equation (1) that if the length (L) and thethickness (t) of the light conductor 1 are determined, the uniformity ofthe brightness distribution of emitted light within the light emittingface will be dependent on the emission rate (α). The emission rate (α)of the light conductor 1 having a thickness of tmm, can be calculatedfrom the following equation (2):

α=(1−10^(K))×100  (2)

K can be calculated by measuring the brightness at 20 mm intervals fromthe light incident face end of the light conductor 1 and calculating thegradient (K(mm⁻¹)) from the logarithmic graph representing therelationship between the ratio (L′/t) of the distance (L′) from thelight incident face end to the thickness (t) of the light conductor 1and the brightness thus measured.

In the present invention, a dispersion rate (R%) represented by thefollowing equation (3) is used as a criterion for the uniformity of thebrightness distribution to estimate and consider the uniformity of thebrightness distribution in the surface light source device. Thedispersion rate (R%) is measured as follows. That is, the brightness onthe light emitting face of the light conductor 1 is measured on asubstantially central area of the light conductor 1, the area extendingfrom a point of 5 mm interval far away from the light incident face end,by 20 mm increments to the end portion confronting the light incidentface end of the light conductor 1. The central area is positioned atsubstantially central portion relative to lengthwise direction of thelight source 2. Then, the maximum value (Imax) of the measuredbrightness, the minimum value (Imin) of the measured brightness, and theaverage value (Iav) of the measured brightness are calculated.Thereafter, the dispersion rate (R%) is calculated from the followingequation (3):

R% ={(Imax−Imin)/Iav}×100  (3)

As a result, it has been found that the emission rate (α) and thedispersion rate (R%) are dependent on the length (L) and the thickness(t) of the light conductor 1, and satisfy the specific relationshiptherebetween. That is, as the emission rate (α) increases, thedispersion rate (R%) also increases. If the emission rate (α) isconstant, the dispersion rate (R%) increases when the ratio (L/t) of thelength (L) and the thickness (t) of the light conductor 1 increases.That is, in the light conductor 1 having a fixed size, the uniformity(dispersion rate) of the brightness distribution within the lightemitting face of the light conductor 1 is dependent on the emission rate(α) of the light conductor 1, and good uniformity of the brightnessdistribution can be achieved by controlling the emission rate (α).

In addition, the inventors of the present application have also foundthat in the case where the surface (light emitting face 12, back surface13) of the light conductor 1 is designed to have a fine uneven roughenedsurface comprising many fine convex members having substantiallyspherical surface, or to have many lens arrays which extend parallel tothe light incident face 11 of the light conductor 1, the emissiondirection and emission rate of the light emitted from the lightconductor 1 and the emission rate vary in accordance with the gradientof the uneven portion constituting the roughened surface or the gradientof the slant surfaces constituting the lens arrays.

Particularly in the case of the fine uneven roughened surface, the fineuneven shape of the roughened surface can be approximated by a slantsurface having a gradient. Here, the average slant angle (θa) which isdefined by ISO 4287/1-1987 may be used as the gradient. As the averageslant angle (θa) increases, the light emitted from the light conductor 1becomes substantially parallel to the direction normal to the lightemitting face 12, i.e. the direction of the thickness t of the lightconductor 1. Further, as the average slant angle (θa) increases, theemission rate (α) of the light emitted from the light conductor 1 alsoincreases. Therefore, the uniformity of the brightness distributionwithin the light emitting face 12 of the surface light source device canbe enhanced by lowering the emission rate (α) of the light from thelight conductor 1, that is, the uniformity can be more enhanced byreducing the average slant angle (θa).

On the basis of the above new knowledge, according to the presentinvention, at least one of the light emitting face of the lightconductor 1 or the back surface of the light conductor 1 which confrontsthe light emitting face is designed to have a roughened surface or aplurality of lens arrays having an average slant angle (θa) of 0.5 to7.5 degrees. With this design, the emission rate (α) of the lightemitted from the light conductor 1 can be sufficiently reduced, and thusgood uniformity of brightness distribution within the light emittingface 12 of the surface light source device can be achieved. If theaverage slant angle (θa) of the roughened surface is less than 0.5degrees, the total amount of the light emitted from the light emittingface 12 of the light conductor 1 decreases so that sufficiently highbrightness cannot be obtained, or, the light emission angle of the lightemitted from the light emitting face (the angle relative to the normalto the light emitting face 12) increases, and thus the emitted lightcannot be directed toward the normal direction even by using an anglevarying member such as a prism sheet, lens sheet or the like. On theother hand, if the average slant angle (θa) exceeds 7.5 degrees, thelight emission rate (α) of the light conductor 1 increases, and thus theuniformity of the brightness distribution of the surface light sourcedevice is lowered. Preferably, the average slant angle (θa) is setwithin a range from 1 to 6 degrees, and more preferably it is set withina range from 2 to 5 degrees.

The average slant angle (θa) on the roughend surface having the fineuneven shape can be calculated as follows. First, the surface roughnessof the roughened surface which is formed on the surface of the lightconductor 1 is measured at a driving speed of 0.03 mm/second by a probetype (tracing) surface roughness tester, subtracting an average linefrom the measured chart to correct the slant, and then calculating theaverage slant angle (θa) from the following equations (4) to (5). Here,L″ represents a distance which is scanned by the probe, x represents ameasurement position and f(x) represents a displacement of the probe:

Δa=(1/L″)∫₀ ^(L″)|(d/dx) f(x)|(dx)  (4)

θa=tan⁻¹ Δa  (5)

Furthermore, according to the present invention, in order tosufficiently enhance the uniformity of the brightness distribution ofthe surface light source device, it is preferable to set the emissionrate (α) of the light emitted from the light emitting face 12 of thelight conductor 1 to 1% to 4.5%. If the light emission rate (α) from thelight emitting face of the light conductor is less than 1%, the lightemission angle of the light emitted from the light emitting face (theangle relative to the normal of the light emitting face 12) increases,so that it is increasingly difficult to sufficiently direct the emittedlight toward the normal direction even by using an angle varying membersuch as a prism sheet or the like. Conversely, if the emission rate (α)exceeds 4.5%, the uniformity of the brightness distribution of thesurface light source device of the liquid crystal display device or thelike trends to be lowered. Preferably, the emission rate (α) is set to1% to 4%, and more preferably it is set to 1.2% to 3.5%.

In order to achieve good uniformity of the brightness distribution inthe surface light source device, it is preferable that the followinglight emission characteristic be obtained. That is, that the lightemitted from the light emitting face of the light conductor 1 bedirected from the light emitting face so that the peak light (i.e.,having the maximum light intensity) is emitted at an angle of 65 degreesor more with respect to the normal direction to the light emitting face,or that the intersecting angle between the direction of the peak lightand the direction of the light having half (50%) of the maximum lightintensity is equal to 20 degrees or less. If the peak light having themaximum light intensity is emitted at an angle of less than 65 degreesto the normal of the light emitting face, or if the intersecting anglebetween the direction of the peak light having the maximum lightintensity and the direction of the light having the half (50%) maximumintensity exceeds 20 degrees, the emission rate (α) of the light emittedfrom the light conductor 1 increases, and thus it is increasinglydifficult to achieve good uniformity of brightness distribution on thelight emitting face.

Still furthermore, in the present invention, a brightness which is ashigh as possible is needed for the surface light source device, and itis preferable that the light emitted from the surface light sourcedevice is concentrated in an observation (viewing) direction. Therefore,it is preferable that the intersecting angle between the direction ofthe peak light having the maximum intensity from the light conductor 1and the direction of the light having a light intensity of 10% of themaximum light intensity be below 50 degrees. If the intersecting angleexceeds 50 degrees, the amount of light which is emitted in a directionother than the observation direction increases even by using a lightangle varying sheet, so that a sufficiently high brightness cannot beobtained.

In the present invention, when the surface of the light conductor 1comprises a fine uneven roughened surface comprising a plurality of fineconvex members each having a substantially spherical surface, it ispreferable to make the radius of curvature of the convex membersuniform, and also it is preferable to satisfy a specific relationshipamong the average period (P), the minute average radius of curvature (R)and the average deviation (S) of the distribution of the minute averageradius of curvature of the fine convex members constituting theroughened surface of the light conductor 1. That is, it is preferablethat the ratio (R/P) of the minute average radius of curvature (R) andthe average period (P) of the convex members be set to 3 to 10, and thatthe ratio (S/R) of the average deviation (S) of the distribution of theminute average radius of curvature and the minute average radius ofcurvature (R) be set to 0.85 or less.

If the ratio (R/P) of the minute average radius of curvature (R) of theconvex members and the average period (P) of the convex members is lessthan 3, the average slant angle (θa) of uneven surfaces of the convexmembers having the substantially spherical surface increases, and thusthe emission rate (α) of the light conductor 1 also increases.Therefore, the uniformity of the brightness distribution within thelight emitting face of the light conductor 1 tends to be lowered. On theother hand, if the ratio (R/P) is more than 10, the average slant angle(θa) of the uneven surface of the convex members having substantiallyspherical surface is reduced, and thus the emission rate (α) of thelight conductor 1 is excessively reduced. Therefore, the total amount oflight emitted from the light emitting face of the light conductor 1 isreduced and thus sufficient brightness cannot be obtained. Preferably,the ratio (R/P) is set to 5 to 7.

Furthermore, if the ratio (S/R) of the average deviation (S) of thedistribution of the minute average radius of curvature and the minuteaverage radius of curvature (R) exceeds 0.85, the distribution of theconvex members formed on the surface of the light conductor 1 is madedisuniform, and the uniformity of the brightness distribution within thelight emitting face of the light conductor 1 tends to be lowered.Preferably, the ratio (S/R) is set to 0.8 or less, and more preferablyto 0.7 or less.

In the present invention, the average period (P) of the convex membersis defined as follows. That is, a linear area having a fixed distance(for example, 1000 micrometers) in any direction on the roughenedsurface of the light conductor 1 is measured by a surface roughnesstester to measure the number of crests of the convex members, and theaverage period (P) is obtained as the average value of the period whichis calculated on the basis of the measured number of crests (e.g., theaverage period P is equal to the fixed distance (for example, the 1000micrometers) divided by the number of crests). Further, the minuteaverage radius of curvature (R) can be calculated from a chart which isobtained by measuring the roughened surface comprising the convexmembers with a surface roughness tester as follows. First, forsimplicity, the spherical shape of the convex members is simplified asor assumed to be a circular arc. As shown in the coordinate system ofFIG. 3, a circle is represented by the following equation (6) where rrepresents the radius of the circle:

y ²−2ry+x ²=0  (6)

By solving equation (6) for y, the following equation (7) is obtained onthe assumption that the projecting portion of each of the convex membersis directed in a negative direction of y:

y=x ² /{r+(r ² −x ²)^(½)}  (7)

As commonly used in the optical lens design field, when the centerportion of the spherical surface of the convex member is used, r>x, andthe following equation (8) is obtained as an approximate version ofequation (7):

y=x ²/2r  (8)

Further, the spherical surface (circular arc) can be substituted by aquadratic curve as follows:

d ² y/dx ²=1/r  (9)

Therefore, it is found that the secondary differential coefficient isequal to the reciprocal of the radius. Accordingly, the minute averageradius of curvature (R) can be calculated by calculating the secondarydifferential coefficient from the chart curve measured by the surfaceroughness tester, and then by calculating the average value of thereciprocal of the secondary differential coefficient.

Further, if an area is divided into n equal sub-areas, and if the radiusof curvature of each sub-area is represented by r_(i), then the minuteaverage radius of curvature (R) can be found by the following equation(10): $\begin{matrix}{R = \frac{\sum\limits_{i = 1}^{n}r_{i}}{n}} & (10)\end{matrix}$

The average deviation (S) of the minute average radius of curvature (R)shows a deviation from the average value, and thus it is represented bythe following equation (11): $\begin{matrix}{S = \frac{\sum\limits_{i = 1}^{n}{{r_{i} - R}}}{n}} & (11)\end{matrix}$

In the present invention, the minute average radius of curvature (R) andthe average deviation (S) are measured when the size of the minutesub-area is set below 5 micrometers. The ratio (S/R) of the averagedeviation (S) of the distribution of the minute average radius ofcurvature and the minute average radius of curvature (R) is representedby the following equation (12): $\begin{matrix}{{S/R} = \frac{\sum\limits_{i = 1}^{n}{{r_{i} - R}}}{nR}} & (12)\end{matrix}$

In order to enhance the brightness of the surface light source device,it is preferable to concentrate the light emitted from the surface lightsource device in the observation (viewing) direction, and thus it ispreferable to concentratedly emit the light in one direction from thelight conductor 1. According to the present invention, in order toconcentratedly emit the light in one direction from the light conductor1, the roughened surface constituting at least one of the light emittingface of the light conductor and the back surface thereof is preferablydesigned so that an area having a minute average slant angle (Δθa) of 20degrees or more is located with an occupation rate (or density) of 2% orless on the roughened surface. If the occupation rate (or density) ofsuch an area exceeds 2%, the degree of concentration of the lightemitted from the light conductor 1 is lowered, and the rate of the lightwhich is emitted in a direction other than the observation (viewing)direction is increased even by using a light angle varying member suchas a prism sheet or the like therewith in combination, with the resultthat the brightness of the surface light source device cannot besufficiently enhanced. Preferably, the occupation rate (or density) ofthe area having the minute average slant angle (Δθa) of 20 degrees ormore is set to 1% or less.

Particularly when the emission rate (α) of the light emitted from thelight conductor 1 is lowered, the rate of light which propagates or goesand returns in the light conductor 1 while being reflected is increased,so that the amount of the light emitted from the light conductor 1itself is reduced. Therefore, it is preferable to enhance the lightbrightness of the surface light source device by concentrating the lightemission direction of the emitted light to one direction. The occupationrate of the area having the minute average slant angle (Δθa) of 20degrees or more is calculated as follows. That is, the surface roughnessof the roughened surface of the light conductor 1 is measured at adriving speed of 0.03 mm/second by the probe type surface roughnesstester to obtain a surface roughness chart. The chart thus obtained isdivided into minute areas of n portions (n=L/xo) at a fixed minuteinterval (xo) to calculate the minute average slant angle (Δθa) for eachminute area (the interval xo is taken as the interval between themeasurement points xa and xb) according to the following equation (13),and then a rate of the number of the minute areas having the minuteaverage slant angle (Δθa) of 20 degrees or more with respect to thetotal number of minute areas is found:

Δθa=tan⁻¹((f(xa)−f(xb))/xo)  (13)

Further, when the surface of the light conductor 1 is roughened byforming the fine uneven portions, the haze value thereof is preferablyset to 20% to 40%. The reason is as follows. The surface light sourcedevice can provide a brightness having high uniformity and a smalldispersion rate (R%) by reducing the emission rate (α) of the lightemitted from the light emitting face of the light conductor 1. However,when the emission rate (α) is relatively small as described above, therate of light which goes and returns while being reflected in the lightconductor 1 is increased, and the amount of the light emitted from thelight conductor 1 is reduced. Therefore, it is preferable to enhance thebrightness of the surface light source device. Therefore, a surfaceroughening treatment is performed so that the haze value of the lightconductor 1 is set to 20% to 40%, whereby the brightness of the surfacelight source device can be enhanced. If the haze value of the lightconductor 1 is less than 20%, the unevenness of the roughened surface isreduced, and the brightness of the surface light source device cannot besufficiently enhanced. On the other hand, if the haze value exceeds 40%,the unevenness of the roughened surface is disadvantageous, and spotsare liable to occur in the emitted light or the uniformity of thebrightness distribution tends to be reduced. Preferably, the haze valueis set to from 30% to 40%.

The processing method of uniformly forming a plurality of fine convexmembers having substantially spherical surface on the light conductor 1is not limited to any specific one. For example, there may be used amethod of transferring a roughened surface by a heat-press method, aninjection molding method or the like by using a mold or die made ofmetal or glass on which a roughened surface is formed by a chemicaletching method using hydrofluoric acid or the like, a mold or die whichis roughened by blasting with fine particles such as glass beads or thelike, or a mold or die on which a roughed surface is formed by using theblasting and chemically etching methods in combination, a method ofunevenly coating or sticking transparent materials onto the lightconductor 1 by a printing method or the like, a method of directlyprocessing the light conductor 1 by a blasting method or an etchingmethod, or the like. Of these methods, the following method ispreferable. That is, fine particles such as glass beads or the like areblown onto the surface of a glass plate to perform a blasting treatmenton the glass plate, and then the blast-treated surface of the glassplate is subjected to chemical etching with hydrofluoric acid or thelike to form a roughened surface on the glass plate (i.e., a mold or diehaving the roughened surface). By using the mold or die thus formed, theroughened surface is transferred onto a transparent plate by theheat-press method or the like, or transparent resin is injected into themold, whereby the light conductor 1 having the roughened surface isformed.

The plural lens arrays to be formed on the surface of the lightconductor 1 are not limited to specific ones insofar as the lens arraysare designed to have slant surfaces having an average slant angle (θa)of 0.5 to 7.5 degrees as shown in FIGS. 4 and 5. For example, alenticular lens array having an arcuate shape in section, a prism arrayhaving a saw-toothed shape in section, an uneven array having acontinuous wavelike shape in section or the like are all possible. Ofthese arrays, the prism array (FIG. 4) and the lenticular lens array(FIG. 5) which are symmetrical on the right and left sides in sectionare particularly preferable. Such lens arrays are formed so as to extendin parallel to the light incident face 11 of the light conductor 1, andmore preferably the lens arrays are formed so as to be continuous to andparallel to one another. The pitch of the lens arrays is suitablyselected in accordance with the application thereof, and normally it ispreferably set to 20 micrometers to 5 mm.

As the processing method of forming the plural lens arrays comprisingthe slant surfaces having a specific average slant angle (θa) on thesurface of the light conductor 1, there may be used a method ofperforming a heat-press on a transparent substrate or performing theinjection molding of transparent resin by using a mold or die made ofmetal or glass on which a lens pattern is formed by a chemical etchingmethod, a tool cutting method, a laser processing method or the like, amethod of coating a transparent substrate with resin which can be curedby activation energy irradiation, and then curing the resin by applyingthe activation energy irradiation thereby transferring a lens pattern, amethod of directly processing the light conductor 1 by an etchingmethod, a tool cutting method, a laser processing method or the like.

The size of the light conductor 1 of the surface light source deviceaccording to the present invention is not limited to specific one.However, in order to enhance the advantageous effect of the presentinvention even more, the ratio (L/t) of the length (L) and the thickness(t) of the light conductor 1 is preferably set to 200 or less. If L/texceeds 200, the uniformity of the brightness distribution within thelight emitting face is not sufficiently achieved even by reducing theaverage slant angle (θa) of the roughened surface or of the lens arraysof the light conductor 1. Preferably, L/t is set to 150 or less.Particularly when the surface light source device is used for a liquidcrystal display device, L/t is preferably set to 100 or less, and morepreferably set to 80 or less.

In the present invention, a transparent planar member of glass orsynthetic resin may be used as the light conductor 1. As syntheticresin, there may be used various kinds of highly transparent syntheticresins such as acrylic resin, polycarbonate resin, vinyl chloride resin,etc. These resins may be molded into a planar member by a normal moldingmethod such as the extrusion molding method, an injection molding methodor the like to form a light conductor. Particularly, methacrylate resinis excellent in light transmission, heat resistance, dynamiccharacteristics and molding and processing performance, etc., and thusit is more suitable as the material for the light conductor.Particularly, resin containing methyl methacrylate units of 80% byweight or more is preferable as the methacrylate resin. Further,inorganic fine particles such as glass beads, titanium oxide or thelike, or fine particles made of styrene resin, acrylic resin, siliconeresin or the like may be dispersed as light-diffusing material in thelight conductor 1.

In the surface light source device of the present invention, the lightsource 2 such as a fluorescent lamp or the like is disposed adjacent toone end portion (light incident face 11) of the light conductor 1described above, and a reflection layer 4 of a reflection film or thelike is formed on the back surface 13 of the light conductor 1confronting the light emitting face 12. In order to effectively guidethe light from the light source 2 to the light conductor 1, the lightsource 2 and the light incident face 11 of the light conductor 1 arecovered by a case or a film 5 which is coated with a reflection agent onthe inside thereof. Further, various shapes such as a planar shape, awedge shape as shown in FIG. 6 (the thickness t gradually decreasesalong L′ direction), a shape as shown in FIG. 7 (the thickness tgradually decreases at both end portions along L′ direction toward thecentral portion), etc., may be adopted for the light conductor 1.

In the surface light source device according to the present invention,the light is normally emitted from the light conductor 1 with such adirectivity that the emission direction thereof is at an angle of 60 to80 degrees to the normal of the light emitting face 12. Therefore, inorder to vary the emission direction of the light to a specificdirection such as in the normal direction or the like, a lightdeflecting sheet or light angle varying sheet 3 is mounted on the lightconductor 1. In this case, a diffusion sheet, a lens sheet having a lensface on which a plurality of lens units are formed in parallel on atleast one surface thereof or the like may be used as the light anglevarying sheet 3. The shape of the lenses formed on the lens sheet variesin accordance with the particular application purpose. For example, aprism shape, a lenticular lens shape, a wavelike shape or the like maybe used. The pitch of the lens units of the lens sheet is preferably setto about 30 micrometers to 0.5 mm. When a prism sheet is used, the apexangle of the prism is suitably determined on the basis of thepredetermined emission angle of the light emitted from the lightconductor 1, and generally it is preferably set to 50 to 120 degrees.Further, the direction of the lens sheet is suitably determined on thebasis of the predetermined emission angle of the light emitted from thelight conductor 1. The prism sheet may be mounted so that the lens faceis disposed at the light conductor side or the opposite side.

In the light conductor 1 which has a roughened surface or surfacecomprising the plural lens arrays having the specific average slantangle (θa), the prism sheet having the apex angle of 50 to 75 degrees isusually mounted so that the prism face is disposed at the lightconductor side, whereby the light emitted from the light emitting facecan be directed substantially in the normal direction with respect tothe light emitting face 12.

In the surface light source device according to the present invention, aplurality of light angle varying sheets 3 are usable when overlaid uponeach other. For example, when two lens sheets are used, these lenssheets may be stacked so that the lens arrays of these lens sheetsintersect at an angle or are parallel to one another. Each of the lenssheets may be disposed with the lens face thereof laid face up or down.Further, the lens sheets may be disposed so that the lens faces thereofare disposed at opposite sides. In this case, it is preferable that thefirst lens sheet adjacent to the light conductor 1 be disposed so thatthe lens face thereof is located at the light conductor side and thelens arrays 31 are disposed parallel to the light source (see FIG. 1)while the second lens sheet is disposed so that the lens face thereof islocated at the opposite side to the light conductor and the lens arraysthereof are perpendicular to the lens arrays of the first lens sheet.When the prism sheet is used as the lens sheet, it is further preferablethat the apex angle of the first prism sheet be set to 50 to 75 degrees,and the apex angle of the second prism sheet be set to 80 to 100degrees.

Furthermore, according to the surface light source device of the presentinvention, the lens sheet is preferably formed of material having a hightransmission to visible radiation and a relatively high refractiveindex. For example, acrylic resin, polycarbonate resin, vinyl chlorideresin, activation energy curable resin or the like may be used. Of thesematerials, the activation energy curable resin is preferably used fromthe viewpoint of abrasion resistance, ease of handling, productivity,etc. Additive agents may be added to the lens sheet, such asantioxidants, ultraviolet ray absorbent, yellowing preventing agent,blueing agent, pigment, dispersing agent or the like. An extrusionmolding method, an injection molding method or any other normal moldingmethod may be used to manufacture the lens sheet. When the lens sheet 3is manufactured by using activation energy curable resin, a lens portionmade of the activation energy curable resin is formed on a transparentsubstrate such as a transparent film or sheet which is formed oftransparent resin such as polyester resin, acrylic resin, polycarbonateresin, vinyl chloride resin, polymethacryl imide resin, polyolefineresin or the like. First, activation energy curable resin liquid isinjected into a lens mold on which a predetermined lens pattern isformed, and then it is overlaid on the transparent substrate.Subsequently, activation energy such as ultraviolet rays, electron beamsor the like are irradiated through the transparent substrate to theactivation energy curable resin liquid to polymerize and cure the resinliquid, and the cured resin is exfoliated from the lens mold to form alens sheet.

According to the surface light source device of the present invention,together with the lens sheet as described above, there may be used adiffusion sheet, a color filter, a polarizing membrane, or variousoptical elements which can optically deflect, converge or diffuse thelight or vary the optical characteristics thereof. Further, a generallinear pipe type fluorescent lamp may be used as the light source 2.When it is difficult to exchange or replace the light source 2, a lightline comprising plural optical fibers may be used to guide light fromanother light source which is separately disposed.

If a liquid crystal display element 7 is mounted at the light emittingface side of the surface light source device thus constructed as shownin FIG. 8, it can be used as a liquid crystal display device for aportable personal computer, a liquid crystal television or the like. Insuch a liquid crystal display device, very high uniformity is requiredfor the brightness distribution, and it is required to reduce thedispersion rate (R%) to 30% or less, preferably to 25% or less, and morepreferably to 20% or less.

Further, instead of mounting the liquid crystal display element 7, bymounting a signboard on which characters, figures, photographs or thelike are formed on a semi-transparent plastic plate comprised ofmethacryl plate or the like by cutting, printing or the like, it may beused as a sign display apparatus such as a guide signboard, alarge-scale signboard or the like in a station, public facilities or thelike. In such a sign display apparatus, it is required to reduce thedispersion rate (R%) to 250% or less, preferably to 200% or less.

Further, instead of mounting the liquid crystal display element 7, bymounting a signboard on which a traffic guide, a traffic sign or thelike is formed on a plastic plate comprising a methacryl plate or thelike by cutting, printing or the like, it may be used as a traffic signdisplay apparatus for various guide signs, traffic signs, etc., in ahighway road or a general road. In such a traffic sign displayapparatus, it is required to reduce the dispersion rate (R%) to 450% orless, preferably to 300% or less.

Next, the present invention will be described in more detail with thefollowing Examples and Comparative Examples, wherein each physicalproperty and characteristic were measured as follows.

Emission Rate (α)

The brightness was measured at every interval of 20 mm increments fromthe light incident face end of the light conductor, and the gradient(K(mm⁻¹)) of the logarithmic graph which shows the relationship betweenthe ratio (L′/t) of the distance (L′) from the light incident face endto the thickness (t) of the light conductor 1 and the brightness wascalculated to calculate the emission rate (α) from equation (2).

Dispersion Rate (R%)

The light brightness on the light emitting face of the light conductor 1was measured on a substantially central area of the light conductor 1,the area extending from a point of 5 mm interval far away from the lightincident face end by 20 mm increments to the end portion confronting thelight incident face end of the light conductor 1. The central area ispositioned at substantially central portion relative to a directionparallel to the light incident face. The maximum value (Imax) of themeasured brightness, the minimum value (Imin) of the measuredbrightness, and the average value (Iav) of the measured brightness werecalculated. Thereafter, the dispersion rate (R%) was calculated fromequation (3).

Average Slant Angle (θa)

The average slant angle was measured according to ISO4287/1-1987. Thesurface roughness of the roughened surface was measured at a drivingspeed of 0.03 mm/second by a probe type surface roughness tester(SURFCOM 570A produced by Tokyo Seiki Co., Ltd.) using an E-DT-SO4A (1micrometer R, 55° circular cone, diamond) as a probe. A chart wasobtained, and a slant correction was performed by subtracting theaverage line. The average slant angle was calculated from equations (4)and (5).

Measurement of Angular Distribution of Light Emitted from LightConductor

A cold cathode tube was connected to a DC power source through aninverter (CXA-M10L produced by TDK), and a potential of DC 12V wasapplied to the cathode tube to turn on the cathode tube. The lightconductor was mounted on a measuring table so as to be rotatable at thecenter portion thereof around the rotational shaft parallel to the axisof the cathode tube. Subsequently, a black sheet having a pinhole of 3mm in diameter was fixed onto the light conductor so that the pinholewas disposed at the center of the light conductor, and a luminance meter(nt−1° produced by Minolta) was suitably disposed while adjusting thedistance between the luminance meter and the light conductor so that themeasurement circle was set to 8 to 9 mm in diameter. After waiting foraging of the cold cathode tube over a 30 minute period, the rotationalshaft was rotated from +85 degrees to −85 degrees every 1 degree tomeasure the brightness of the emitted light by the luminance meter.

On the basis of the measurement results, the angle (a) of the directionof the peak light having a maximum light intensity with respect to thenormal, the intersection angle (b) between the direction of the peaklight having the maximum light intensity and the direction of the lighthaving half (50%) of the maximum light intensity, and the intersectionangle (c) between the direction of the peak light having the maximumlight intensity and the direction of the light having 10% of the maximumlight intensity were measured as shown in FIG. 9.

Measurement of Brightness in Normal Direction (Normal Brightness) ofSurface Light Source Device (Compact-size Surface Light Source Device)

A cold cathode tube was connected to a DC power source through aninverter (CXA-M10L produced by TDK), and a potential of DC 12V wasapplied to the cathode tube to turn on the cathode tube. The surfacelight source device was mounted on a measuring table so as to berotatable at the center portion thereof around the rotational shaftparallel to the axis of the cathode tube. Subsequently, a black sheethaving a pinhole of 3 mm in diameter was fixed onto the light conductorso that the pinhole was disposed at the center of the light conductor,and a luminance meter (nt−1° produced by Minolta) was suitably disposedwhile adjusting the distance between the luminance meter and the surfacelight source device so that the measurement circle was set to 8 to 9 mmin diameter. After waiting for aging of the cold cathode tube over a 30minute period, the rotational shaft was set to 0 degrees, and thebrightness of the emitted light was measured by the luminance meter. Themeasurement was conducted on the surface light source device, except foran area within 5 mm from the edge of the light conductor confronting thecold cathode tube. The area to be measured was sectioned into squaresub-areas of 20 mm×20 mm in area, and the brightness was measured at thecenter of each square sub-area. Thereafter, the respective measurementvalues were averaged to obtain the normal brightness.

Measurement of Normal Brightness of Surface Light Source Device(Large-size Surface Light Source Device)

Except that a fluorescent lamp of 30 W was used as the light source, thesame measurement method as for the compact-size surface light sourcedevice was used.

Occupation Rate of Area having a Minute Average Slant Angle (Δθa) Above20 Degrees

The surface roughness of the roughened surface was measured in the samemanner as the average slant angle (θa). The chart thus obtained wasdivided into n minute areas at 1 mm intervals, and the minute averageslant angle (Δθa) in each minute area was calculated from equation (13).On the basis of this calculation result, the occupation rate (ordensity) of the minute areas in which the minute average slant angle(Δθa) was above 20 degrees was found as the ratio of the number of theminute areas of Δθa of above 20 degrees to the total number of theminutes areas.

Measurement of Surface Roughness

The measurement of the surface roughness was performed at a drivingspeed of 0.03 mm/second by a probe type surface roughness tester(SURFCOM 570A produced by Tokyo Seiki Co., Ltd.) using a 1-micrometer R,55° circular cone diamond needle (010-2528) as a probe. The measurementvalues (surface roughness) were recorded at an interval of 5micrometers. Furthermore, the primary differential coefficient (Ki) andthe secondary differential coefficient (Li) were calculated on the basisof the measurement values (Di) according to equations (14) and (15):

Ki=(D _(i+1) −Di)/5  (14)

Li=(K _(i+1) −Ki)/5  (15)

Average Period (S)

The primary differential coefficient (Ki) on a linear area of 1000micrometers in any direction of the light conductor was measured at aninterval of 5 micrometers. The primary differential coefficients (Ki)thus obtained were successively linked to one another, and from thefollowing equation (16), the average period (S) was calculated on thebasis of the frequency m at which the link thus obtained traversed the“0” level.

S=(1000×2)/m  (16)

Minute Average Radius of Curvature (R)

The absolute value of the reciprocal of the secondary differentialcoefficient (Li) which was obtained by the probe type surface roughnesstester was calculated, and values thus obtained, except for values whichare less than 10⁻⁶, were averaged. The average value thus calculated wasset as the minute average radius of curvature (R).

Average Deviation (S) of Distribution of Minute Average Radius ofCurvature

From equation (11), the average deviation (S) was calculated from theradius of curvature (ri) and the minute average radius of curvature (R)which were obtained at an interval of 5 micrometers on the linear areaof 1000 micrometers in any direction of the light conductor 1.

EXAMPLE 1

The surface of a glass plate was subjected to a blast treatment usingglass beads of 125 to 149 micrometers in particle size (FGB-120 producedby Fuji Manufacturing Works Co., Ltd.) under the condition that thedistance between the glass plate and a blast nozzle was set to 10 cm andthe blast pressure was set to 4 Kg/cm². Thereafter, a hydrofluoric acidtreatment was conducted to chemically etch the blast surface of theglass plate, and a replica mold or die was obtained by conducting anelectroforming method on the glass plate. By performing a heat-pressusing the mold, the roughened surface thereof was transferred onto onesurface of a transparent acrylic resin plate of 4 mm in thickness and165 mm×210mm in area to form a light conductor.

The roughened surface of the light conductor thus constructed had astructure such that fine convex members having substantially sphericalsurface were uniformly distributed. The average slant angle (θa) and theoccupation rate of the areas having the minute average slant angle (Δθa)of 20 degrees or more of the roughened surface were measured. Theresults are shown in Table 1. The roughened surface of the lightconductor thus obtained was measured by the probe type surface roughnesstester to obtain the roughened surface chart shown in FIG. 10. Theprimary differential coefficient and the secondary differentialcoefficient were calculated from the chart, and the calculation resultis also shown in FIG. 10. Table 1 shows the structure parameters for thelight conductor surface. Further, the angular distribution of the lightemitted from the light conductor was measured to obtain the angle (peakangle) of the peak light having a maximum light intensity to the normal,the intersecting angle (peak 50% angle) between the direction of thepeak light having the maximum light intensity and the direction of thelight having 50% of the maximum light intensity, and the intersectingangle (peak 10% angle) between the direction of the light having themaximum light intensity and the direction of the light having 10% of themaximum light intensity. The results are shown in Table 1.

A PET film on which silver was deposited was adhesively attached to eachof one end surface of 210 mm and two end surfaces of 165 mm of the lightconductor thus obtained, and further a PET film on which silver wasdeposited was fixed to the back surface confronting the light emittingface by an adhesive tape to form reflection surface. A linear pipe typefluorescent lamp (KC230T4E (4 mm in diameter×230 mm in length) producedby Matsushita Electric Co., Ltd.) was disposed at the remaining endsurface of 210 mm of the light conductor. Subsequently, a prism sheetincluding a plurality of parallel prism arrays each having an apex angleof 63 degrees and a pitch of 50 micrometers (which was formed byultraviolet ray curable acrylic resin having a refractive index of 1.53on the PET film) was disposed on the light emitting face of the lightconductor so that the prism face confronted the light emitting face sideof the light conductor, thereby fabricating a surface light sourcedevice. The normal brightness of the surface light source device thusfabricated was measured, and the measurement results are shown in Table1.

A light conductor was formed with a transparent acrylic resin plate of 3mm in thickness and 90 mm×300 mm in area in the same manner as describedabove. A PET film on which silver was deposited was adhesively attachedto each of the two 300 mm end surfaces of the light conductor thusobtained, and a PET film on which silver was deposited was fixed to theback surface confronting the light emitting face by an adhesive tape toform reflection surface. A linear pipe type fluorescent lamp (KC130T4E—4mm in diameter×130 mm in length—produced by Matsushita Electric Co.,Ltd.) was disposed at one end surface of 90 mm of the light conductor.The emission rate and the dispersion rate (R%) of the light conductorthus obtained were measured, and the results are shown in Table 1.

EXAMPLE 2

The surface of a mirror-polished stainless steel plate was subjected tothe blast treatment using glass beads of 125 to 149 micrometers inparticle size (FGB-120 produced by Fuji Manufacturing Works Co., Ltd.)under the condition that the distance between the stainless steel plateand a blast nozzle was set to 10 cm and the blast pressure was set to 4Kg/cm². By performing a heat-press using this stainless steel plate moldor die, the roughened surface was transferred onto one surface of atransparent acrylic resin plate of 3 mm in thickness and 165 mm×210 mmin area to form a light conductor.

The roughened surface of the light conductor thus constructed had astructure such that the fine convex members having substantiallyspherical surface were uniformly distributed. The average slant angle(θa) and the occupation rate of the areas having the minute averageslant angle (Δθa) of 20 degrees or more of the roughened surface weremeasured. The results are shown in Table 1. The roughened surface of thelight conductor thus obtained was measured by the probe type surfaceroughness tester, and the structure parameters of the surface of thelight conductor are shown in Table 1. Further, the angular distributionof the light emitted from the light conductor was measured to obtain theangle (peak angle) of the peak light having a maximum light intensity tothe normal, the intersecting angle (peak 50% angle) between thedirection of the peak light having the maximum light intensity and thedirection of the light having 50% of the maximum light intensity, andthe intersecting angle (peak 10% angle) between the direction of thelight having the maximum light intensity and the direction of the lighthaving 10% of the maximum light intensity. The results are shown inTable 1.

A surface light source device was fabricated with the light conductorthus obtained in the same manner as in Example 1. The normal brightnessof the surface light source device thus obtained was measured, and themeasurement results are shown in Table 1. Further, by using the surfacelight source device which was constructed with the above light conductorin the same manner as Example 1, the emission rate and the dispersionrate (R%) of the light conductor were measured, and the results areshown in Table 1.

EXAMPLE 3

By using the stainless steel plate mold used in Example 2, the roughenedsurface was transferred onto one surface of a transparent acrylic resinplate of 4 mm in thickness and 165 mm×210 mm in area by a thermaltransfer method to obtain a light conductor. The light conductor thusobtained had the same structure, physical properties and characteristicsas that of Example 2. A surface light source device was fabricated withthe light conductor thus obtained in the same manner as the Example 1.The normal brightness of the surface light source device thus obtainedwas measured, and the measurement results are shown in Table 1. By usingthe surface light source device which was constructed with the abovelight conductor in the same manner as Example 1, the emission rate andthe dispersion rate (R%) of the light conductor were measured, and theresults are shown in Table 1.

EXAMPLE 4

A light conductor was obtained in the same manner as in Example 1,except that a wedge-shaped plate having a thickness of 3 mm at one 210mm end and a thickness of 1 mm at another 210 mm end was used as thetransparent acrylic resin plate. The light conductor thus obtained hasthe same structure, physical properties and characteristics asExample 1. Further, a surface light source device was fabricated withthe light conductor thus obtained in the same manner as Example 1 exceptthat the linear pipe type fluorescent lamp was disposed at the endsurface side having the thickness of 3 mm of the light conductor. Thethe normal brightness of the surface light source device thus fabricatedand the emission rate (α) and dispersion rate (R%) of the lightconductor thereof were measured, and the results are shown in Table 1.

Comparative Example 1

A light conductor was obtained in the same manner as Example 2 exceptthat glass beads of 74 to 88 micrometers in particle size (FGB-200produced by Fuji Manufacturing Works, Co., Ltd.) were used for the blasttreatment. The average slant angle (θa) and the occupation rate of areashaving the minute average slant angle (Δθa) of 20 degrees or more of thelight conductor thus obtained were measured, and the results are shownin Table 1. The roughened surface of the light conductor thus obtainedwas measured by the probe type surface roughness tester to obtain theroughened surface chart shown in FIG. 11. The primary differentialcoefficient and the secondary differential coefficient were calculatedfrom the chart, and the calculation result is also shown in FIG. 11.Table 1 shows the structure parameters of the light conductor surface.Further, the angular distribution of the light emitted from the lightconductor was measured to obtain the angle (peak angle) of the peaklight having a maximum light intensity with respect to the normal, theintersecting angle (peak 50% angle) between the direction of the peaklight having the maximum light intensity and the direction of the lighthaving 50% of the maximum light intensity, and the intersecting angle(peak 10% angle) between the direction of the light having the maximumlight intensity and the direction of the light having 10% of the maximumlight intensity. The results are shown in Table 1.

Like Example 1, a surface light source device was fabricated with thelight conductor thus obtained. The normal brightness of the surfacelight source device thus obtained was measured, and the results areshown in Table 1. Further, by using the surface light source devicewhich was constructed with the above light conductor in the manner asExample 1, the emission rate and the dispersion rate (R%) of the lightconductor were measured, and the measurement results are shown in Table1.

Comparative Example 2

By using the stainless steel plate mold used in Comparative Example 1,the roughened surface was transferred onto one surface of a transparentacrylic resin plate of 4 mm in thickness and 165 mm×210 mm in area bythe thermal transfer method so as to form a light conductor. The lightconductor thus obtained had the same structure, physical properties andcharacteristics as Example 2. A surface light source device wasfabricated with the light conductor in the same manner as Example 1. Thenormal brightness of the surface light source device thus obtained wasmeasured, and the measurement results are shown in Table 1. Further, byusing the surface light source device which was constructed with theabove light conductor in the same manner as Example 1, the emission rateand the dispersion rate (R%) of the light conductor were measured, andthe results are shown in Table 1.

Comparative Example 3

A light conductor was obtained in the same manner as Example 2 exceptthat glass beads of 53 to 62 micrometers in particle size (FGB-300produced by Fuji Manufacturing Works, Co., Ltd.) were used for the blasttreatment and the blast pressure was set to 5 Kg/cm². The average slantangle (θa) and the occupation rate of areas having a minute averageslant angle (Δθa) of 20 degrees or more of the light conductor thusobtained were measured, and the results are shown in Table 1. Further,the roughened surface of the light conductor thus obtained was measuredby the probe type surface roughness tester, and the structure parametersof the light conductor surface were as shown in Table 1. Further, theangular distribution of the light emitted from the light conductor wasmeasured to obtain the angle (peak angle) of the peak light having themaximum light intensity to the normal, the intersecting angle (peak 50%angle) between the direction of the peak light having the maximum lightintensity and the direction of the light having 50% of the maximum lightintensity, and the intersecting angle (peak 10% angle) between thedirection of the light having the maximum light intensity and thedirection of the light having 10% of the maximum light intensity. Theresults are shown in Table 1.

The normal brightness of the surface light source device thus obtainedwas measured, and the measurement results are shown in Table 1. Further,by using the surface light source device which was constructed with theabove light conductor in the same manner as Example 1, the emission rateand the dispersion rate (R%) of the light conductor were measured, andthe results are shown in Table 1.

EXAMPLE 5

A prism pattern comprising a plurality of prism arrays of 172 degrees inapex angle and 50 micrometers in pitch which were formed continuously inparallel to one another corresponding to the shape shown in FIG. 4, wasformed on a brass plate by using a diamond cutter to form a die or mold.By using the die thus obtained, a prism face was transferred onto onesurface of a transparent acrylic resin plate of 4 mm×210 mm×165 mm insize by the thermal transfer method to form a light conductor. Theaverage slant angle (θa) of the light conductor thus obtained was equalto 4.2 degrees. A silver-deposited PET film was adhesively attached toeach of the two 165 mm end surfaces of the light conductor and one ofthe other two end surfaces of the light conductor, and asilver-deposited PET film was fixed to the back surface confronting thelight emitting face having the prism face by an adhesive tape to formreflection surface. Further, a cold cathode tube (KC230T4E—4 mm indiameter×230 mm in length, produced by Matsushita Electric Co., Ltd.)used as a light source was disposed on the remaining end surface of thelight conductor by covering with a silver-deposited PET film. Aplurality of parallel prism arrays were formed of ultraviolet raycurable acrylic resin having a refractive index of 1.53 on a PET film soas to have an apex angle of 63 degrees and a pitch of 50 micrometers toform a prism sheet. The prism sheet thus formed was disposed on thelight emitting face of the light conductor so that the prism faceconfronted the light emitting face side of the light conductor, therebyfabricating a surface light source device. The normal brightness of thesurface light source device thus fabricated was measured, and themeasurement results are shown in Table 2.

Further, a light conductor was formed in the same manner as describedabove by using a transparent acrylic resin plate of 3 mm×90 mm×300 mm insize. The surface light source device was fabricated in the same manneras described above, except that a silver-deposited PET film wasadhesively attached to each of two 300 mm end surfaces of the lightconductor thus constructed. The emission rate and the dispersion rate(R%) of the light conductor of the surface light source device weremeasured, and the measurement results are shown in Table 2.

EXAMPLE 6

A lens pattern comprising a plurality of lenticular lens arrays of 50micrometers in pitch which were formed continuously in parallel to oneanother corresponding to the shape shown in FIG. 5, was formed on abrass plate by using a diamond cutter to form a die. By using the diethus obtained, a lenticular lens face was transferred onto one surfaceof a transparent acrylic resin plate of 4 mm×210 mm×165 mm in size bythe thermal transfer method to form a light conductor. The average slantangle (θa) of the light conductor thus obtained was equal to 4.3degrees. A surface light source device was fabricated with the lightconductor thus obtained in the same manner as in Example 5. The normalbrightness of the surface light source device thus obtained wasmeasured, and the measurement results are shown in Table 2. Further, byusing the surface light source device which was constructed with theabove light conductor in the same manner as Example 5, the emission rateand the dispersion rate (R%) of the light conductor of the surface lightsource device were measured, and the measurement results are shown inTable 2.

Comparative Example 4

A prism pattern comprising a plurality of prism arrays of 164 degrees inapex angle and 50 micrometers in pitch which were formed continuously inparallel to one another corresponding to the shape shown in FIG. 12, wasformed on a brass plate by using a diamond cutter to form a die. Byusing the die thus obtained, a prism face was transferred onto onesurface of a transparent acrylic resin plate of 4 mm×210 mm×165 mm insize by the thermal transfer method to form a light conductor. Theaverage slant angle (θa) of the light conductor thus obtained was equalto 8.2 degrees. A surface light source device was fabricated with thelight conductor thus obtained in the same manner as in Example 5. Thenormal brightness of the surface light source device thus obtained wasmeasured, and the measurement results are shown in Table 2. Further, byusing the surface light source device which was constructed with theabove light conductor in the same manner as Example 5, the emission rateand the dispersion rate (R%) of the light conductor the surface lightsource device were measured, and the measurement results are shown inTable 2.

Comparative Example 5

A lens pattern comprising a plurality of lenticular lens arrays of 50micrometers in pitch which were formed continuously in parallel to oneanother corresponding to the shape shown in FIG. 13, was formed on abrass plate by using a diamond cutter to form a die. By using the diethus obtained, a lenticular lens face was transferred onto one surfaceof a transparent acrylic resin plate of 4 mm×210 mm×165 mm in size bythe thermal transfer method to form a light conductor. The average slantangle (θa) of the light conductor thus obtained was equal to 8.3degrees. A surface light source device was fabricated with the lightconductor thus obtained in the same manner as in Example 5. The normalbrightness of the surface light source device thus obtained wasmeasured, and the measurement results are shown in Table 2. Further, byusing the surface light source device which was constructed with theabove light conductor in the same manner as Example 5, the emission rateand the dispersion rate (R%) of the light conductor of the surface lightsource device were measured, and the measurement results are shown inTable 2.

As a comparative test, the surface light source devices obtained inExamples 1 to 6 and Comparative Examples 1 to 5 were used as back lightsfor a liquid crystal display device. In the case of the surface lightsource devices of Examples 1 to 6, a very light and uniform liquidcrystal display image frame was obtained. On the other hand, in case ofthe surface light source devices of Comparative Examples 1 to 5, arelatively light image frame was observed in the neighborhood of thelight source. However, the reduction in lightness becamedisadvantageously more noticeable farther away from the light source.

EXAMPLE 7

In the same manner as Example 1, the roughened surface was transferredonto one surface of a transparent acrylic resin plate of 10 mm thick and600 mm×1250 mm in area by the thermal transfer method to form a lightconductor. The average slant angle (θa) and the occupation rate of theareas having a minute average slant angle (Δθa) of 20 degrees or more ofthe light conductor thus formed were measured, and the measurementresults are shown in Table 3. The structure and the characteristic ofthe light emitted from the light conductor were the same as inExample 1. A silver-deposited PET film was adhesively attached to one600 mm end surface of the light conductor and to two 1250 mm endsurfaces of the light conductor, and a silver-deposited PET film wasfixed to the back surface confronting the roughened light emitting faceby an adhesive tape to form reflection surface. Further, a 30Wfluorescent lamp (FSL30T6W, produced by Matsushita Electric Co., Ltd.)was disposed on the remaining 600 mm end surface of the light conductor.A plurality of parallel prism arrays were formed of ultraviolet raycurable acrylic resin having a refractive index of 1.53 on a PET film soas to have an apex angle of 63 degrees and a pitch of 50 micrometers,thereby forming a prism sheet. The prism sheet thus formed was disposedon the light emitting face of the light conductor so that the prism faceconfronted the light emitting face side of the light conductor, therebyfabricating a surface light source device. The normal brightness of thesurface light source device thus fabricated was measured, and themeasurement results are shown in Table 3.

Further, a light conductor was formed in the same manner as describedabove by using a transparent acrylic resin plate of 10 mm thick and 600mm×1250 mm in area. A silver-deposited PET film was adhesively attachedto each of two 1250 mm end surfaces of the light conductor, and asilver-deposited PET film was fixed to the back surface confronting theroughened light emitting face by an adhesive tape to form reflectionsurface. Further, a 30 W fluorescent lamp (FSL30T6W, produced byMatsushita Electric Co., Ltd.) was disposed on one 600 mm end surface ofthe light conductor. By using the surface light source device thusfabricated, the emission rate and the dispersion rate (R%) of the lightconductor were measured, and the measurement results are shown in Table3.

Comparative Example 6

In the same manner as Comparative Example 1, the roughened surface wastransferred onto one surface of a transparent acrylic resin plate of 10mm in thickness and 600 mm×1250 mm in area by the thermal transfermethod to form a light conductor. The average slant angle (θa) and theoccupation rate of the areas having a minute average slant angle (Δθa)of 20 degrees or more of the light conductor were measured, and themeasurement results are shown in Table 3. The structure of the roughenedsurface of the light conductor and the characteristic of the lightemitted therefrom were the same as in Comparative Example 1. A surfacelight source device was fabricated with this light conductor in the samemanner as in Example 7. The normal brightness of the surface lightsource device thus fabricated was measured, and the measurement resultsare shown in Table 3. By using the surface light source device which wasconstructed with the above light conductor in the same manner as Example7, the emission rate and the dispersion rate (R%)of the light conductorwere measured, and the measurement results are shown in Table 3.

EXAMPLE 8

In the same manner as Example 5, the prism face was transferred onto onesurface of a transparent acrylic resin plate of 10 mm×600 mm×1250 mm insize by the thermal transfer method to form a light conductor. Theaverage slant angle (θa) of the light conductor thus obtained was equalto 4.2 degrees. A silver-deposited PET film was adhesively attached toeach of two 1250 mm end surfaces of the light conductor and to one ofthe other 600 mm end surfaces of the light conductor, and asilver-deposited PET film was fixed to the back surface confronting thelight emitting face having the prism face by an adhesive tape to formreflection surface. Further, a 30 W fluorescent lamp (FSL30T6 producedby Matsushita Electric Co., Ltd.) was disposed on the remaining endsurface of the light conductor and was wrapped by a silver-deposited PETfilm.

A plurality of parallel prism arrays were formed of ultraviolet raycurable acrylic resin having a refractive index of 1.53 on a PET film soas to have an apex angle of 63 degrees and a pitch of 50 micrometers,thereby forming a prism sheet. The prism sheet thus formed was disposedon the light emitting face of the light conductor so that the prism faceconfronted the light emitting face side of the light conductor, therebyfabricating a surface light source device. The normal brightness of thesurface light source device thus fabricated was measured, and themeasurement results are shown in Table 4.

Further, a light conductor was formed in the same manner as describedabove by using a transparent acrylic resin plate of 10 mm×600 mm×1250 mmin size. The surface light source device was fabricated in the samemanner as described above, except that a silver-deposited PET film wasadhesively attached to each of two 1250 mm end surfaces of the lightconductor. By using the surface light source device thus fabricated, theemission rate and the dispersion rate (R%) of the light conductor weremeasured, and the measurement results are shown in Table 4.

EXAMPLE 9

In the same manner as Example 6, the lenticular lens face wastransferred onto one surface of a transparent acrylic resin plate of 10mm×600 mm×1250 mm in size by the thermal transfer method to form a lightconductor. The average slant angle (θa) of the light conductor thusobtained was equal to 4.3 degrees. A surface light source device wasfabricated with the light conductor thus obtained in the same manner asin Example 8. The normal brightness of the surface light source devicethus fabricated was measured, and the measurement results are shown inTable 4. Further, the surface light source device was formed with theabove light conductor in the same manner as in Example 8, and theemission rate and the dispersion rate (R%) of the light conductor of thesurface light source device thus obtained were measured, and themeasurement results are shown in Table 4.

Comparative Example 7

In the same manner as Comparative Example 4, the prism face wastransferred onto one surface of a transparent acrylic resin plate of 10mm×600 mm×1250 mm in size by the thermal transfer method to form a lightconductor. The average slant angle (θa) of the light conductor thusobtained was equal to 8.2 degrees. A surface light source device wasfabricated with the light conductor thus obtained in the same manner asExample 8. The normal brightness of the surface light source device thusfabricated was measured, and the measurement results are shown in Table4. Further, the surface light source device was formed with the abovelight conductor in the same manner as Example 8. and the emission rateand the dispersion rate (R%) of the light conductor of the surface lightsource device thus obtained were measured, and the measurement resultsare shown in Table 4.

Comparative Example 8

In the same manner as Comparative Example 5, the prism face wastransferred onto one surface of a transparent acrylic resin plate of 10mm×600 mm×1250 mm in size by the thermal transfer method to form a lightconductor. The average slant angle (θa) of the light conductor thusobtained was equal to 8.3 degrees. A surface light source device wasfabricated with the light conductor thus obtained in the same manner asin Example 8. The normal brightness of the surface light source devicethus fabricated was measured, and the measurement results are shown inTable 4. Further, the surface light source device was formed with theabove light conductor in the same manner as in Example 8, and theemission rate and the dispersion rate (R%) of the light conductor of thesurface light source device thus obtained were measured, and themeasurement results are shown in Table 4.

A semi-transparent acrylic plate on which a photograph was printed andalternately a semi-transparent acrylic plate on which a traffic sign wasprinted were disposed on the surface light source device obtained inExamples 7 to 9 and Comparative Examples 6 to 8 to fabricate alarge-size signboard or traffic sign apparatus. In the signboards andthe traffic sign apparatus which were formed by using the surface lightsource devices of Examples 7 to 9 of the present invention, the imagewas very light and uniform over the whole frame. On the other hand, inthe signboards and the traffic sign apparatus which were formed by usingthe surface light source devices of Comparative Examples 6 to 8, theimage was relatively light in the neighborhood of the light source.However, the lightness of the image was quite reduced farther from thelight source, and the image was very dark in the neighborhood of the tipend portion of the surface light source device.

Industrial Applicability

According to the present invention, the roughened surface whichcomprises a plurality of substantially spherical fine convex membershaving an average slant angle (θa) of 0.5 to 7.5 degrees, or a pluralityof lens arrays having slant surfaces whose average slant angle (θa) isequal to 0.5 to 7.5 degrees, is formed on at least one of the lightemitting face of the light conductor or the back surface of the lightconductor which confronts the light emitting face. Therefore, thesurface light source device of the present invention can provide lighthaving high brightness and a uniform brightness distribution within thelight emitting face without performing any uniformity treatment using aspot pattern or the like. Therefore, the surface light source deviceaccording to the present invention can be suitably applied to a liquidcrystal display device for a portable personal computer, a liquidcrystal television or the like, or to a display apparatus such as aguide marking board or a large-size signboard in a station or otherpublic facilities, and to a guidepost or traffic sign on a highway roador a general road.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Com. Ex. 1 Comp. Ex. 2 Com. Ex. 3 L/t41.3 55 41.3 — 55 41.3 55 AVERAGE SLANT ANGLE 2.7 2.9 2.9 2.7 8.4 8.421.8 (θ a) (Degree) RATE OF AREA HAVING 0 0.5 0.5 0 3 3 4 Δ θ a ABOVE20° (%) DISPERSION RATE 14 19 18 18 163 121 850 (R %) (%) EMISSION RATE(α) 1.27 1.73 1.73 2.50 4.67 4.67 8.46 (%) NORMAL BRIGHTNESS 2424 20741991 2450 2324 2291 2060 (cd/cm²) MINUTE AVERAGE 206.6 255.8 255.8 206.669.0 69.0 49.8 RADIUS OF CURVATURE (R) (μm) AVERAGE DEVIATION 135.0196.7 196.7 135.0 61.1 61.1 44.9 (S) (μm) S/R 0.657 0.769 0.769 0.6570.886 0.886 0.902 AVERAGE PERIOD (P) 35.1 48.8 48.8 35.1 28.6 28.6 37.0(μm) R/P 5.86 5.20 5.20 5.86 2.41 2.41 1.35 PEAK ANGLE (Degree) 71 70 7071 63 63 67 ANGULAR DIFFERENCE 15 16 16 15 26 26 23 OF 50% LIGHTINTENSITY (Degree) ANGULAR DIFFERENCE 32 47 47 32 51 51 62 OF 10% LIGHTINTENSITY (Degree)

TABLE 2 Ex. 5 Ex. 6 Com. Ex. 4 Com. Ex. 5 L/t 41.3 41.3 41.3 41.3AVERAGE SLANT ANGLE 4.2 4.3 8.2 8.3 (θ a) (Degree) DISPERSION RATE 17 18110 115 (R %) (%) EMISSION RATE (α) 1.61 1.61 4.15 4.15 (%) NORMALBRIGHTNESS 2303 2327 2176 2240 (cd/cm²)

TABLE 3 Ex. 7 Com. Ex. 6 L/t 125 125 AVERAGE SLANT ANGLE 2.7 8.4 (θ a)(Degree) RATE OF AREA HAVING 0 3 Δ θ a ABOVE 20° (%) DISPERSION RATE 180650 (R %) (%) EMISSION RATE (a) 3.40 9.10 (%) NORMAL BRIGHTNESS 397 345(cd/cm²) MINUTE AVERAGE 206.6 69.0 RADIUS OF CURVATURE (R) (μm) AVERAGEDEVIATION 135.0 61.1 (S) (μm) S/R 0.657 0.886 AVERAGE PERIOD (P) 35.128.6 (μm) R/P 5.86 2.41 PEAK ANGLE (Degree) 71 63 ANGULAR DIFFERENGE 1526 OF 50% LIGHT INTENSITY (Degree) ANGULAR DIFFERENCE 32 51 OF 10% LIGHTINTENSITY (Degree)

TABLE 4 Com. Com. Ex. 8 Ex. 9 Ex. 7 Ex. 8 L/t 125 125 125 125 AVERAGESLANT ANGLE 4.2 4.3 8.2 8.3 (θ a) (Degree) DISPERSION RATE 170 180 630670 (R %) (%) EMISSION RATE (α) 3.20 3.20 8.10 8.30 (%) NORMALBRIGHTNESS 352 360 308 315 (cd/cm²)

What is claimed is:
 1. A surface light source device comprising: a lightsource; a light conductor which has a light incident face on at leastone side end surface thereof which confronts said light source, and alight emitting face on one surface thereof which is substantiallyperpendicular to said light incident face; and a light angle varyingsheet which is disposed at a side of said light emitting face of saidlight conductor, wherein at least one of said light emitting face and aback surface of said light conductor comprises a minute structure havingan average slant angle of 0.5 to 7.5 degrees relative to an average linedefined by said one of said light emitting face and said back surface.2. The surface light source device as claimed in claim 1, wherein saidlight angle varying sheet comprises a lens sheet having a plurality oflenses which are formed parallel to one another on at least one surfacethereof.
 3. The surface light source device as claimed in claim 2,wherein said lens sheet is a prism sheet having a plurality of prismswhich are formed parallel to one another on at least one surfacethereof.
 4. The surface light source device as claimed in claim 1,wherein said minute structure comprises a roughened surface whichincludes a plurality of fine convex members each having a substantiallyspherical surface, and said roughened surface includes areas which havea minute average slant angle of 20 degrees or more at an occupation rateof 2% or less.
 5. The surface light source device as claimed in claim 1,wherein said minute structure comprises a plurality of lens arrayshaving slant surfaces which extend parallel to said light incident faceand which have an average slant angle of 0.5 to 7.5 degrees.
 6. Thesurface light source device as claimed in claim 5, wherein said lensarrays comprise prism arrays.
 7. The surface light source device asclaimed in claim 5, wherein said lens arrays comprise lenticular lensarrays each having an arcuate shape in section.
 8. The surface lightsource device as claimed in claim 1, wherein the light emission ratefrom said light emitting face of said light conductor is set to 1% to4.5%.
 9. The surface light source device as claimed in claim 1, whereina peak light having a maximum light intensity which is emitted from saidlight emitting face of said light conductor is emitted at an angle of 65degrees or more with respect to a normal to said light emitting face.10. The surface light source device as claimed in claim 1, wherein anintersecting angle between a direction of a peak light having a maximumintensity emitted from said light emitting face of said light conductorand a direction of light having 50% of the maximum light intensity isequal to 20 degrees or less.
 11. The surface light source device asclaimed in claim 1, wherein said light conductor is designed so that aratio (L/t) of a length (L) from the light incident face to an end faceconfronting said light incident face and a thickness (t) of said lightconductor is set to 200 or less.
 12. A display apparatus, comprising thesurface light source device as claimed in claim 1 as a back light. 13.The display apparatus as claimed in claim 12, wherein a dispersion rate(R%) of the brightness of light emitted from said light emitting face ofsaid light conductor is set to 450% or less.
 14. The display apparatusas claimed in claim 13, wherein the display apparatus is a traffic signdisplay apparatus.
 15. The display apparatus as claimed in claim 12,wherein the display apparatus is a sign display apparatus.
 16. Thedisplay apparatus as claimed in claim 15, wherein the dispersion rate(R%) of the brightness of light emitted from said light emitting face ofsaid light conductor is set to 250% of less.